This application relates generally to processing microelectromechanical systems (MEMS), and more particularly to postprocessing of MEMS dice.
In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (MEMS) and are desirable because they may be constructed with considerable versatility despite their very small size. Much of the development of MEMS processes has taken advantage of developments in the semiconductor processing industry, although there are significant differences.
One difference is that semiconductor processing is typically performed on the scale of wafers. In particular, semiconductor devices are typically formed by a series of deposition and etching steps on a silicon wafer having a diameter of 100 mm, 200 mm, or 300 mm. Many devices are thus formed simultaneously with a single wafer so that economies can be realized from the batch processing. By contrast, in MEMS processing, only a portion of the process is performed at the wafer scale. A MEMS device is formed through initially depositing structural and sacrificial layers on a wafer, both of which may be etched in specific ways as part of the process. Subsequently, the structures are diced using a 75- to 250-xcexcm-wide diamond or carbide saw blade and the MEMS devices are xe2x80x9creleasedxe2x80x9d through immersion in a substance corrosive to the sacrificial layers. Typically the sacrificial layers are formed of silicon oxide, which is dissolved through immersion of individual dice in hydrofluoric acid (HF). Each die, after immersion in hydrofluoric acid, is then rinsed, for example in a bath of deionized water. Because these processes are performed individually, MEMS processing does not realize the same economies of scale available in semiconductor processing. Additionally, since the release and rinses are typically performed by holding an individual die with tweezers, there is a significant risk of accidental damage to the delicate device.
This risk of accidental damage is further increased because MEMS devices are generally not passivated prior to the release and rinse steps in the formation process. This is in contrast to semiconductor devices, which are generally passivated to promote the formation of a resistant layer that provides some level of protection during handling. Since the MEMS devices are unpassivated, they are especially sensitive to surface contact that may damage the silicon layers used to form the device. In addition, for the same reasons, they are more susceptible to harmful effects from debris and contamination than are semiconductor devices.
After the release and rinsing steps, the MEMS dice are packaged. Because these post-dicing processing steps cannot take advantage of the large-scale integration used in the semiconductor-processing industry, their cost may be considerable. It is not unusual for the post-dicing processing of MEMS devices to account for as much as 80% of the overall cost of a component or system. There is accordingly a need in the art for a method and apparatus that provides for batch processing of MEMS dice and that limits contact with a human handler.
Thus, embodiments of the invention provide a method for processing a plurality of MEMS dice, such as may be prepared by dicing a processed wafer. The plurality of MEMS dice are secured in a holder. A process step is then performed on the plurality of MEMS dice while they are secured in the holder. Such a process step may be performed simultaneously on all of the MEMS dice secured in the holder. In one embodiment, the plurality of MEMS dice include unreleased MEMS dice.
A variety of processing steps may be performed. For example, the holder may be immersed in one or more liquids, thereby immersing all of the MEMS dice secured by the holder in the liquid. One liquid may be hydrofluoric acid, which is corrosive to silicon oxide layers commonly used as sacrificial material in fabricating MEMS devices, so that the immersion in hydrofluoric acid acts to release the MEMS devices. The holder may thus be made of a material resistant to corrosion by hydrofluoric acid, such as the fluoropolymer resin teflon(copyright). Another liquid may be used to rinse hydrofluoric acid after release, such as deionized water. Another processing step that may be performed while the MEMS dice are secured in the holder is critical point drying to prevent damage to the delicate structures after rinsing. Other process steps that may be performed include testing the plurality of MEMS dice, as well as some steps involved in packaging the MEMS dice.
In one embodiment, the holder comprises a structural body having a plurality of stations. Each station is adapted to secure a MEMS die. In one embodiment, this is achieved with a recess in the structural body shaped to secure an edge of the MEMS die together with a flexible retaining arm adapted to retain the MEMS die within the recess. The flexible retaining arm may include a notch shaped for engagement with a tool for flexing the flexible retaining arm. Each station may also include an access to an underside of the MEMS die, as may be provided with a hole or slot in the structural body. In one embodiment, the structural body is circularly symmetric and the plurality of stations are configured symmetrically about a central axis of the structural body. The structural body may be formed as a single continuous structure.